`dpkg` - Debian package manager, one level below apt

dpkg [-l] [-L PKG] [-S PATH] [-i FILE.deb]

Options you will use

-l [PATTERN]: List installed packages.
-L PKG: List the files a package installed.
-S PATH: Which package owns this file.
-i FILE.deb: Install a .deb directly (it does NOT resolve dependencies).
-V, --verify [PACKAGE]: Check installed files against the package checksums (changed config and binaries show up).

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